A conventional air blowing device used for a ventilating fan mounted to a ceiling is described hereinafter. This blowing fan includes a housing having a suction port, a blow-off port, and a fan disposed within the housing. The fan includes a fan case, an impeller disposed in the fan case, and a motor for driving the impeller. The fan case includes a fan inlet communicating with the suction port and a fan outlet communicating with the blow-off port.
The fan case defined around the impeller, more specifically defined in an area up to a sectional face cut along a line between a tongue section and an opposite section to the tongue section, is shaped such that: a distance between the fan case and the impeller increases gradually along the rotary direction of the impeller. This structure is disclosed in, e.g. Patent Literature 1: Unexamined Japanese Patent Application Publication No. H04-41996.
However, the foregoing conventional device is obliged to accept a low efficiency of the motor. To be more specific, since the conventional blowing device is mounted to a ceiling as a ventilating fan, a rate of rotation of the impeller is kept low in order to lower an operating sound, i.e. a noise level. To compensate the lower rate of rotation, a size of the impeller is obliged to be larger so that a given performance can be achieved. The greater size of the impeller causes the greater load to the motor as a matter of course, thereby lowering the efficiency of the motor.